Impoundments are used for the storage and disposal of chemicals and wastes. The use of impoundments has increased dramatically over the recent past considering the present waste disposal problems. The possibilities presented by biological treatment of hazardous waste further contribute to the need for the creation and use of impoundments. Liquid impoundments are made by scooping out a pond or lake, typically using earth-moving equipment, and a geomembrane is spread across the pond. These geomembrane liners (often called flexible liners) are large sheets of plastic or rubber material used as a barrier so as to contain the liquids within the impoundment. Facilities where these liners are commonly used include hazardous waste landfills and liquid impoundments, water reservoirs, and other surface impoundments.
In certain types of facilities, such as hazardous liquid surface impoundments, the geomembrane liner is often comprised of two separate layers of liner material to provide an additional margin of containment. The two-layer geomembrane liner consists of a lower plastic sheet typically made of high-density polyethylene or other polymeric material, a water-permeable intermediate material made of loosely woven or fusion-bonded plastic stringers (or, in some cases, a sand layer of several inches thickness), and an upper plastic sheet similar to the lower sheet. The grade of the bottom of the impoundment is constructed with an incline to accommodate a sump drain to collect any leakage from the impoundment through the upper liner and into the interliner zone.
Previous methods for monitoring the performance of liners after installation and use have typically been based on ground water sampling using a plurality of monitoring wells at spaced intervals along the perimeter of the impoundment. The ground water sampling method, however, provides only an indirect and delayed indication of leakage and, therefore, is not adequate for monitoring liner performance since ground water contamination may take years to occur. Furthermore, by the time a leak has been determined by this method, substantial ground water contamination may have already occurred.
Another source of inadequacy in the ground water sampling method stems from the need to have the monitoring well in the particular aquifer which is transporting the contaminants. An adequate ground water monitoring program, therefore, requires a large number of monitoring wells along the perimeter of the impoundment with a sufficient number of wells sampling water from different levels within the various aquifers beneath the impoundment. Even the most elaborate ground water monitoring system, however, cannot provide monitoring as accurately and timely as necessary.
One non-intrusive method for detecting and locating leaks in geomembrane liner systems is described in U.S. Pat. No. 4,725,785, issued on Feb. 16, 1988, to Converse et al. This method uses an electrical measurement technique which takes advantage of the high electrical insulating properties of the liner with respect to the liquid contained above the liner and the soil underneath the liner. In general, geomembrane liners made from an impervious plastic material or rubber have a very high electrical resistance. A liner installed in a landfill or liquid impoundment, therefore, effectively acts as an electrical insulator between the materials contained within the facility and the surrounding environment. If the integrity of the liner is lost due to a puncture or separation, however, conductive liquid may then flow through the leak, thus establishing an electrical shunt through the liner between the contained liquid and the conductive earth in surrounding contact with the liner. The shunt is a low resistance path for current flow which forms an electrically detectable region corresponding to a leak which may be detected and located.
U.S. Pat. No. 4,755,757, issued on Jul. 5, 1988, to J. W. Cooper describes a system and method for measuring the leakage flow rate from an impounded liquid through a tear or hole in the geomembrane liner. This system utilizes a narrowed passage connected with an inverted funnel to confine the flow, and further includes forming a transverse magnetic field thereacross, a sensor mutually perpendicular to the passage and the magnetic field and a volt meter connected to the sensor for measuring the voltage. The voltage is dependent on the rate of flow of the leaked liquid across the passage.
U.S. Pat. No. 4,751,841, issued on Jun. 21, 1988, to Biard et al. describes a detector for use in determining the rate of loss of liquid from an impoundment. This device includes first and second open ended columns for receiving the impounded liquid therein. The first column is isolated while the second column is communicated near the bottom thereof with a passage into the impoundment so that its height will fall with the height of liquid in the impoundment. A measuring means is interposed between the two columns to measure differences in height. The two columns are equipped with baffles to suppress column height agitation.
U.S. Pat. No. 4,751,467, issued on Jun. 14, 1988, to J. W. Cooper describes another method and apparatus for determining the rate of flow of a leak through a geomembrane. A surrounding lower skirt having a peripheral weight thereabout is placed on the bottom to surround the location of the leak. The skirt supports a cover. A portion of the skirt or cover is made of an ionic and electrically permeable membrane to permit current flow. A second liquid is defined so as to be miscible with the first liquid and have a markedly different electrical conductivity. The rate of flow of the second liquid out of the lower skirt and cover is determined by measuring the electrical potential between the liquid in the impoundment and the second liquid within the skirt as the electrical conductivity of the surrounding earth is altered by invasion of the second liquid into the soil under the geomembrane liner. The apparatus utilizes a reservoir of the second liquid which is delivered through a suitable valve and fill hose into the lower skirt cover.
U.S. Pat. No. 4,905,210, issued on Feb. 27, 1990, to T. E. Owen describes a liquid impoundment survey vehicle that incorporates a position finding and tracking system. A set of hydrophones spaced around the impoundment detects a liquid transmitted acoustic pulse from the vessel. The vessel supports an RF transmitter which broadcasts a timing pulse. With the timing pulse elapsed acoustic travel time is measured to define range in the liquid. The range defines an arc of a circle around a single hydrophone.
U.S. Pat. No. 4,740,757, issued on Apr. 26, 1988, to Converse et al. describes a method and apparatus for locating leaks in a multiple layer geomembrane liner. In this apparatus, in the event of a tear or perforation formed in the lower liner, a current flow path is established from a power supply and conductors connected to the power supply. The current flow path extends through the liquid to the leak. Because of the liquid path through the liner, electrical current will flow through the perforation and establish an associated magnetic field in the near vicinity of the leak. Magnetic sensors are then swept across the surface of or through the impounded liquid above the liners to indicate such magnetic field variations and the locations of such perturbations corresponding to the location of the leak perforations.
U.S. Pat. No. 4,720,669, issued on Jan. 19, 1988, to T. E. Owen describes a geomembrane liner leak assessment shell shaped probe. This determines the size of a leak in a geomembrane liner by measuring the electric current density through the liner at locations suspected to contain a leaking penetration. By comparing the observed current flow through the liner, as measured by the assessment probe, with simulated current conducting contacts, the equivalent cross-sectional area of the leak perforation may be determined.
Finally, U.S. Pat. No. 4,543,525, issued on Sep. 24, 1985, to Boryta et al. shows a method for determining a leak in a pond liner of electrically insulating sheet material. This method immerses a power electrode in an electrically conductive fluid within the pond and a second or ground electrode in the supporting medium so as to create therewith an electrical potential between the pond fluid and the supporting medium. A detector, having a pair of spaced probe electrodes which are electrically connected, is introduced to the fluid near the power electrode with the ends of the probe electrodes adjacent the liner. The detector is rotated until a maximum current reading is obtained by a galvanometer electrically connected to the detector probes. With the probes so aligned with respect to the power electrode, the probes are caused to traverse the liner and the location of a leak is noted by a sharp change in the galvonometer reading as one of the probes passes over the leak.
It is an object of the present invention to provide a leak detector that is suitable for creating a position vector as to the leak location.
It is another object of the present invention to provide an impoundment leak detector that utilizes a sensor array for measuring electrical potentials.
It is a further object of the present invention to provide an impoundment leak detector that is suitable for detecting multiple leaks.
It is a further object of the present invention to provide an impoundment leak detector that is relatively easy to utilize, simple to manufacture, and relatively easy to install.
It is still a further object of the present invention to provide an impoundment leak detector that is non-intrusive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.